Protective surface treatment of magnesium base alloys



A 1949- M G. E. COATES 2,480,448

PROTECTIVE SURFACE TREATMENT OF MAGNESIUM BASE ALLOYS I Filed Feb. 21,1944 2 Sheets-Sheet 1 lo 9 LL] .(0 8 I v F 7 Z 5- a 5 O E 01 I 4- 3 3 Zz z SULPHA TE CONCENTRATION, HOLES. PER L/TRE lo v 5 CORROSION RATE INEMPIRICAL UN/TvS N c, -h. o, 0: \T 00 a Aug. 30, 1949. e. E. COATES ,4

PRO'IECTIVE SURFACE TREATMENT OF'MAGNES IUM BASE ALLOYS Filed Feb. 21,1944 2 Sheets-Sheet 2 Fig. 3,

Nu4x=ncnw om CORROSION RATE IN EMPIRICAL UNITS I /0 20 TIME or TREATMENTIN M/NU 7E6 Patented Aug. 30, 1949 PROTECTIVE SURFACE TREATMENT OFMAGNESIUM BASE ALLOYS Geoffrey Edward Coates, Swansea,-Wales, assignor,by mesne assignments, to The Permanente Metals Corporation, acorporation of Delaware Application February 21, 1944, Serial No.523,295 In Great Britain April 28, 1943 1 Claim. 1

This application, which is a continuation in part of pending UnitedStates application Serial No. 469,444, of Geoffrey Edward Coates, filedDecember 18, 1942, now abandoned, for Treatment of magnesium basealloys, is for an invention which relates to the protective surfacetreatment of magnesium base alloys, especially but not exclusively thosealloys prepared according to specifications of the British Air MinistryDepartment of Technical Development hereinafter referred to as D. T. D.specifications.

The protection afforded by dichromate solutions is due to the formationof a protective film of reaction products on the surface of the metal,but simple dichromate solutions are of little practical value becauseboth the rate and total extent of film formation are severely limited.

Amongst what may be described as the dichromate type of treatments forthe corrosion protection of Mg base alloys, recourse has been made to avariety of methods in endeavors to overcome the above difficulties butno hitherto published process combines all the desired attributes of acommercial protective treatment which may be summarized as thecombination of the following features or requirements:

(a) The treatment should cause the minimum dimensional change so thatmachined articles may be treated without disturbance to machiningtolerances, for instance, less than can be reasonably measured with athousandths of an inch micrometer.

(b) The raw materials for compounding the solution should be cheap andof innoxious character, e. g., strongly acid or alkaline solutionsshould be avoided and the rate of consumption of the ingredients inoperation should be low.

Time of treatment should be as short as possible, i. e., preferably notmore than a few minutes and less than half an hour.

(d) The process should be capable of operation at room temperature.

(6) The process should be characterized by ease and economy in operationand maintenance and the solutions should be stable.

The present invention provides good commercial protection in a shorttime at room temperatures where such protection can be secured even in 1to 3 minutes in certain circumstances. On the other hand, when a highertemperature is used for speeding up the process, for instance to fit inwith conjoined processes, the bath still remains stable. Heavy metalsalts are avoided because they form heavy metal deposits in the coating,or precipitated or colloidal matter, or

cause spontaneous change of pH with time or form double salts with someofthe solution constituents of insufficient solubility to permitattainment of optimum sulphate concentration. Such salts are thereforeexcluded from the present application, which is' limited to those.compounds of which the cations remain in stable solution in theconcentration and pH ranges set out below.

The object of the present invention is to provide an improved processfor the protection of Mg base alloys by the dichromate method,characterized by, namely, short-time treatment, capacity for treatmentat substantially room temperature, avoidance of strongly acid solutionswhich, in addition to being unpleasant to use, are likely to cause unduedimensional loss and employment of economical stable solutions.

The characteristics of the solutions of this in vention are that, in thepH range specified, the cations are stable in aqueous solution, undergono spontaneous change, are stable in the presence of dichromate ions, donot react with other constituents of the bath to prevent attainment andmaintenance of the sulfate and dichromate ion concentrations specified,and do not react with magnesium or form other metal or undesirabledeposits on the magnesium surface.

The new combination consists of the following essential requirements (a)to (e), namely:

(a) Dichromate ions in concentration not less than005 molar andpreferably between 0.2 and 0.5 molar, which region is the optimum whenthe other factors are at their optima. The most convenient sources ofdichromate ions to ensure that the cation simultaneously introducedshall satisfy the above requirements, are the dichromates singly ormixed of H, Li,Na, Mg, K or NH4.

(12) Sulphate ions-In concentration not less than 0.2 molar andpreferably between 0.6 and 1 molar which region is the optimum when theother factors are at their optima. The most convenient sources ofsulphate ions to ensure that the cation simultaneously introduced shallsatisfy the above requirements, are the sulphates of H, Li, Na, Mg, K orNH4 or mixtures or combinations of any of these. The following cationsare also useful in thedichromate constituents, singly or mixed, of mysolutions: tetramethyl ammonium, tetraethyl ammonium, andN-methyl-pyridinium ions. Some of these sulphates have never previouslybeenspecified as constituents of dichromate solutions for Mg protection.sulphates such asAl sulphate, alums, nickel sulphallic, chromiumsulphateor nickel ammonium sulphate which have been specified for knownprocesses are not suitable for the present process because they wouldall contribute cations undesirable in respect of the requirements listedabove and are excluded. It is; believed. that the advantages of thesulfate addition in the present process may be explained as follows: Thesalts employed previously as activator salts contain cations injuriousin my process'due'to; format-ion of precipitated or colloidal matter,and; the activator salts which contain anions other than sulfate do notproduce the desiredi'results. This is because sulfate ions have theproper charge and are of the proper size to diffuse through the thin,first-formed film and enable produc tion of a satisfactorily thickfilml, without. causing pitting, reducing the dichromate; or forminginsoluble reaction products.

I have found that not only is the choice of the sulphate of essentialimportance but likewise its concentration. Figure '1' ispa. graphshowing the relationship between sulphate concentration inmoles perlitre and corrosion rate in empirical units employinguniform treatmenttimes. There is an optimumconcentrationnn the region of 0.6-1 molar..Whereas thereislittleloss inprotection with. higher concentrations,concentrations below this region becomecritical and the protectionrapidly falls off so that at less than about 0.2. M the solutions are ofno practical value.

(c) Ammonium tuna-Thepresence of ammonium ions improves the protectivevalue of the film for any given time oftreatment. Theconcentration issuiiicient ifqthe ammonium salt is used for at least one of the otheringredients (a) or (b). above..

(d) Hydrogenion c01rcentmtz'on.-It has long been known that if theacidity of dichromate' solutions is increased; the rate. of protectivefilm formation is also increased". Unfortunately the rate of.dissolution of metal. from the workpiece surface also increases and;the; permissible increase in acidity is therefore limited by dimensionaltolerances; The. protective value is also affected if the acidity fallsoutside certain limiting ranges for each type of solution. For thesolutions according; to the present invention the eifect of pH variationon the protection afiorded by the film in the case of two. particularalloys is shown in Figure 2., Thereare marked. indications of an optimumpH.-value for each alloy. Generally speaking; the optimum depends on theAlcontent of the alloy as shown in Figure 3 which shows the relationshipbetween the hydrogen ion concentration andpercentage of Al in alloystreated according to certain solutions of the present invention. Thedegree of protection falls ofi rapidly on either side of the optimumvalues for each. alloybut. allvalues fall within the extreme pH- range4-6;

The pH values quotedrwerezdetermined. by the glass electrode method.Other methods of pH measurement may be used but allowance must be madefor variations from the glass electrode values.

(6) Bu-fier anemia-Maintenance of the pH value at the optimum or withinthe permissible range for a given alloy is complicated by the fact thatthe pH- rises as the Mg dissolves during the treatment. In practicaloperation it is highly desirable for reasons of economy, tobe able totreat a number of workpieces in the same batch of solution. Thisnecessitates frequent controlled additions of acid but the frequency ofsuch additions is greatly reduced by incorporating in the solution aweak acid and one of its salts capable of buffering the solution withinthe desired pH range. Acetic acid is about the best and cheapest organicacid for the bufier mixture together with its sodium or ammonium salt,although any of the other bases listed under (1)) above may be used.Other weak acids of suitable dissociation constant and salts thereof,both of suflicient solubility, may be used in place of or in addition tothe acetic acid and acetate. The bufier acid must not however form aninsoluble salt with Mg and must not be oxidized by the dichromatesolution. The buiier capacity or resistance of the solution to change ofpH is rough-- ly proportional to the, concentration of the weak acid andits salt. Occasional major adjustments in pH may be appropriatelyefiected by additions of sulphuric, chromic or nitric acids, ammonia,caustic soda or other suitable bases selected from the list in- (b)above. The acids and their salts which I prefer for cold solutions aresingle or mixed acids and single or mixed salts of the acids andpreferably of the same acids within the following group, namely, acetic,propionic, butyric, valeric, malonic, succinic, glutaric, adipic,pimelic, phthalic, the cations of the salt being those of lithium,sodium, magnesium, potassium or ammonium. The acids and their saltswhich I prefer for hot solutions are single or mixed acids and single ormixed salts ofthese acids and preferably of the same acid, the acidsbeing chosen so that the buffer agent is sufl'iciently soluble at thetemperature-of use, from the following group, namely, caproic,oenanthic, malonic, succinic, glutaric, adipic, suberic, pimelic,benzoic and phthalic, and the cations of the salts being those oflithium, sodium, magnesium, potassium or ammonium. In cases where it isde sired to secure the maximum possible protection, the sulphate ionconcentration should not exceed the optimum region indicated under (1))above, and in such cases the major pH adjustment should be made bychromic or nitric acid additions.

The salts used for introducing the dichromate and sulfate ions, the saltof the buffer acid and the base for pH adjustments must be chosen tomeet the requirements hereinabove set forth, and suitable cations havebeen listed above. Any species of cation can be tested by compounding asolution as described, containing it, and determining whether thesolution filters without leaving appreciable residue on the filterpaper, and whether it shows a change of pH on standmg.

To sum up the aforesaid items (a) to (e), the invention consists in aprocess for the protective surface treatment of magnesium base alloys insolutions characterized by a dichromate concentration not less than 0.05mole per litre, a sulphate concentration not less than 0.2 mole perlitre, and a hydrogen ion concentration controlled by a buiTer agent oraddition within the pH range 4-6; the constituent salts being soselected that within this pH range their cations remain in stablesolution and the upper limits for the dichromate and sulphateconcentrations corresponding to saturation point in the particularsolutions used and temperature of use.

Time of treatment to secure the best results for any particular alloydepends on the composition of the alloy. Generally speaking time oftreatment must be increased with increasing Al content. In some casesthere is an optimum region fromthe point of view of timelof treatment asshown by the shape of curve a in Figure 4. Curve e applies to an alloyof composition 7.5% A1, 0.4% Zn, 0.2% Mn. Curve applies to an alloycomprising 6% Al, 1% Zn,0.2% Mn. The region of maximumcurvature of curvedis about thepoint correspondin'g to, say, 9 minutes." That in curve eis about thepoint corresponding to, say, 7 minutes, although with'thealloy of curve d an increased time of treatment up to 30 minutes is notdetrimentaL'curve e shows that with the alloy of that curve an-increasedtime oftreatment-other things being equal-reduces the protection, thatis to say increases the corrosion rate in respect of the effect obtainedby treatment at a lower time corresponding to the knee of the curve.

In anygiven case, the optimum value for any of the variables may bedetermined by means of accelerated corrosiontests representative of theparticular service i-conditi'onsrthe article is required to withstand. Asaline immersion or spray test applied in well-known manner' 'isgenerally regarded as representative of marine "atmospheric exposure andit is on'this basis that my ranges and the results ini'th'e figureshave'been established.

Variation in the ionic concentrations do not cause sudden changes in.the effectiveness of the solution, and working limits-must therefore beselected having regard'to'the degree of protection required. V

Drag-out loss is minimized by using the lower concentrations in anygiven range, by treating in the cold, and'by suspending the workpieceabove the bath for a few seconds after treatment. The dichromateconcentration tends to fall with use and further additions must be madeas indicated by simple analysis.: The sulphate concentration on theother hand rises when sulphuric acid instead of chromic or nitric acidis'used forth occasional major adjustments'in pI-I.

One additional advantage arising from the fact that good protection issecured by a few minutes treatment in the cold is that it may be appliedas a simple swabbing process to finished parts in situ, e. g. cut edgesof sheet, iwelds, repairs or even as a maintenance treatment. If,nevertheless, the solutions are heated, the time of treatment may be'even' further reduced and for certain applications this may more thanoffset the disadvantages incurred by the heating. Ex-

amples are .where thetreatment is -a step in a continuous productionline and tank capacity is limited by considerations of floor space orwhere output must be temporarily or permanently increased in an existingtreatment plant.

tion loss of acetic acid b-ecomes serious, the acetic acid and acetateshould be substituted in whole or in part by single or mixed acids'andsingle or mixed salts of these acidsand preferably of the same acid,-asdisclosedabove for hot solutions.

For best results, whether the solution is used cold or hot, it isdesirable to submit the work- .pieces to a cleaning pre-treatment.Solvent de- The solutions are stable onheating but if the evapora- Inthe latter case it may be desirable in some cases to extend slightly theprotective treatment time. Alternatively, anodic polishing in accordancewith the methods disclosed in British patent specifications Nos. 550,175and 550,176 may be employed as the pre-treatment.

The following table gives a number of specific examples in accordancewith which the invention may be carired into eifect:

Table I For magnesium alloys substantially Al free and containing up to2.5% manganese in accordance with-specifications D. T. D. 118, 140A and142, a high degree of protection is secured by cleaning by one of themethods described above and immersing for three minutes, at roomtemperature in a solution made up as follows:

Parts by weight Ammonium, sodium or potassium dichromate, singly ormixed 1 50-100 Ammonium sulphate (or, if desired, the equivalent amountof sodium sulphate if the ammonium salt is used for one The pH of thissolution must be adjusted, e. g. with ammonium hydroxide, to between 5.2and 5.9 but preferably to 5.5. After washing the material is foundto becoated with an adherent greenish or gold brownprotective film which alsoprovides an excellent basis for the usual type of paint coating appliedto magnesium alloys.

In cases where a good, but not necessarily the highest, degree ofprotection is required, the treatment timemay be reduced to one minute.Alternatively, the above concentrations may be reduced by as much as 50%and the treatment time kept at 3 minutes.

. For magnesium alloys containing Al to spec,- ifications D.'T. D. 59A,88B, 136A, A, 281, 285, 289, 325, 348 and 350, a very high degree ofprotection is secured by cleaning by one of the methods described aboveand immersing for 8-12 minutes in the same solution but of pH adjustedto suit the A1 content as indicated in Figure 3. .In this figure thecentral curve (1 represents the pH values for maximum protection. Atolerance is provided at each side of this curve as shown by the curvesb and c. The shaded area between the curves 5 and 0 gives theworking-range for good protection. Thus, for an alloy containing 6% Al,.1% Zn and 0.2% manganese the pH range should be 4.8-5.3..but preferably5.0-5.1. For an alloy containing 7.5% A1, 0.4% Zn and 0.2% Mn the pHshould be adjusted to between 4.6 and 5.2

I but preferably between 4.8 and 4.9. After Washminutes.

7 In the following Tables II to VI the pH values may be adjusted asdescribed above.

Table II This table may be taken to comprise the ommended range ofingredients.

Dichromate (CrzO7)--Over 0.05 molar (up to saturation point).

Sulphate (s04)-OVI 0.2 molar. (In cases where the dichromateconcentration is increased, the sulphate Concentration should preferablyfollow suit up to saturation point.)

Bufier mixture, e. g., acetic acid and sodium acetate, total acetateconcentration as acetic acid plus sodium acetate together, over 0.1molar CH3COO-. When other weak acids and their salts are used as buffermixtures, these concentrations must be altered having regard to theequivalent weight of the acid relative to that of acetic acid.

pH 4-6 depending on alloy composition and time of treatment.

Time of treatment-Thi depends on the composition of the alloy, thesolution composition and its pH and varies between /z-3Q minutes.

Table III This table may be taken to comprise the range where the degreeof protection is that corresponding to temporary protection of partsduring fabrication, storage or transit, or the like,

Dichromate (CrzO'z)-Over 0.1 molar (up to saturation point).

Sulphate ($O4)-Over 0.3 molar. (In cases where the dichromateconcentration is increased, the sulphate concentration should preferablyfollow suit up to saturation point.)

Buifer mixture, e. g. acetic acid and sodium acetate, total acetateconcentration as acetic acid plus sodium acetate together, over 0.1molar CH3COO. When other weak acids and their salts are used as buffermixture, these concentrations must be altered having regard to theequivalent weight of the acid relative to that of acetic acid.

pH 4-6 depending on alloy composition and time of treatment.

Time of treatment-As in Table II,

Table IV This table comprises the ranges for parts requiring a goodaverage degree of protection.

Dichromate (Cr2Ow)--0.1 to 0.6 molar.

Sulphate (SO4)-0.4 to 2.0 molar.

Buffer mixture, e. g. acetic acid and sodium acetate, total acetateconcentration as acetic acid plus sodium acetate together, from 0.2molar CHsCOO to 1.0 molar CI-I3COO. When other Weak acids and theirsalts are used as buffer mixtures, these concentrations must be alteredhaving regard to the equivalent weight of the acid relative to that ofacetic acid,

pH 4-6 depending on the alloy composition and time of treatment.

Time of treatment-As in Table II.

Table V This table gives the optimum composition for the protectivevalues of the highest order.

Dichromate (C1'2O'1)0.2 to 0.5 molar.

Sulphate (SOD-0.6 to 1 molar.

Bufier mixture, e g. acetic acid and sodium acetate, total acetateconcentration as acetic recacid plus sodiumacetate: together, 0.2 to 1.0molar CHaCOO. When other weak acids and their salts are used as buffermixtures, these concentrations must be altered having regard to theequivalent weight of the acid relative to that of acetic acid.

Ammonium ions (NH4) 0.4 to 3 molar.

pH 4.6-5.9 depending on the alloy composition and time of treatment.

Time of treatment-L430 minutes depending on the alloy composition.

Major pH adjustments in this case should preferably be made by nitricnot sulphuric acid, to maintain the sulphate ion concentration at theoptimum.

Table VI This represents the effect of increased temperature.

A magnesium alloy containing 7.5% A1, 0.4% Zn and 0.2% Mn is given acleaning pro-treatment as described above and treated for ten minutes inthe optimum solution of Table V containing adipic acid and sodiumadipate and of pH 4.85 (measured in the cold by glass electrode) heatedto (SO- C. After washing, the specimen is found to be coated with ablack adherent film. When tested by a saline test in known manner, theprotective value of this film is found to be of the order of twice thatobtained by treatment at room temperature under the optimum conditions;instead of taking the benefit of the heat by increased protection forthe same time of treatment, it may be taken by similar protection for ashorter time of treatment.

I claim:

A process for the portective surface treatment of magnesium base alloywhich comprises treating the alloy with a solution consisting of water,0.2 to 0.5 mol per liter of dichromate ion of at least one compound ofthe group consisting of the dichromates of lithium, potassium, sodium,magnesium, and ammonium, 0.6 to 1.0 mole per liter of sulfate ion of atleast one compound chosen from the group consisting of the sulfates oflithium, potassium, sodium, magnesium, and ammonium, a bufler mixturecomprising acetic acid and sodium acetate and containing 0.2 to 1.0 moleper liter of acetate ion and adapted to maintain the pH of the solutionat from 4.6 to 5.9. the said dichromate and sulfate compounds being sochosen as to yield an arm- ;ntonium ion concentration of 0.4 to 3 molesper GEOFFREY EDWARD COATES.

REFERENCES CIT ED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,138,794 Nelson et al Nov. 29,1938 2,178,977 Tcsterud Nov. '7, 1939 2,224,245 Allen Dec. 10, 19402,224.528 Sutton et al Dec. 10, 1940 2,288,552 Siebel et al June 30,1942 2,352,076 Bushrod June 20, 1944 FOREIGN PATENTS Number Country Date353,415 Great Britain July 14, 1931 558,983 Great Britain Jan. 31, 1944

